This invention relates to solid state suppressors for suppressing transients and the like, particularly, but not exclusively, at low temperatures.
It is known to provide transient suppressors for protecting all types of equipment, for example, Telecommunications equipment. The suppressors can be based on a thyristor structure which is capable of suppressing high voltages and thereby diverting any surge currents from the equipment.
The following description describes a thyristor structure based on an N-type starting material. Complementary versions of this structure starting with a P-type material are also applicable. A similar description would apply with the interchange of "N" and "P" labels. Additionally, the use of the term N.sup.- generally refers to an N-type material that is lightly doped and N.sup.+ generally refers to an N-type material that is heavily doped as is known in the art.
FIG. 1 shows a typical prior art bi-directional transient suppressor in the form of a thyristor. These devices typically have the following dimensions, 100 mil.times.100 mil, and are fabricated as an array of columns and rows of devices on a single silicon slice. Due to the dimensions of the slice, it is necessary for the silicon to be about 10 mil thick to support the devices and to be sufficiently durable to survive the manufacturing process.
The breakdown voltage of the thyristor is determined by the structure and the conductivity levels of the P-type and N.sup.- type base regions. The conductivity of the N.sup.- type base region usually has the most significant influence on the breakdown voltage. As this region is formed from the slice starting material, some manufacturers select the start material conductivity to set the breakdown voltage. Alternatively, the slice conductivity may be chosen to give a higher breakdown voltage than required. The breakdown voltage is then reduced to the required level by selectively diffusing regions of higher N type conductivity into the N.sup.- type base region immediately adjacent to the P-type base layer. Although the discussion details the N type diffusion method of setting the breakdown voltage, the performance improvements described are equally applicable to the other voltage setting techniques.
From a design point of view, the N.sup.- diffusion region for each half of the thyristor is used to set the breakdown voltage of the suppressor. A typical breakdown voltage would be in the range of 18 to 350 volts. Accordingly, the N.sup.- diffusion region need only be about 1.5 mil to support this voltage and the reverse bias depletion layer. In addition, the P type junctions need only be made to be about 1-2 mil deep to accommodate passivation requirements. Ideally, therefore, the device should be made on a slice having a thickness of about 5 mil. Clearly this has not been practical, since a slice of this thickness is not sufficiently durable to survive the manufacturing process. This results in the use of a thicker slice for the manufacture of thyristor devices which means that the N.sup.- region is generally larger (thicker) than necessary. The use of a thickness greater than necessary for the N.sup.- region has the drawback that the on-state voltage is increased and the switching speed is slower than for a thyristor structure with the minimum N.sup.- region thickness. These drawbacks lead to increased power dissipation under surge conditions and reduced maximum surge rating. In addition, the thick N.sup.- region offers a high resistance until the region is conductivity modulated by injected carriers from the emitter. This can cause high transient voltages to appear across the device before full protection can be established. These transients can cause damage to the circuitry being protected.
It has been proposed to overcome the disadvantages of the above present system by using a thick P.sup.+ substrate for the anode and growing the N.sup.- layer epitaxially. This type of structure, however, will only conduct in one direction and two separate structures are required if transient protection is required with both polarities. This is both costly and inconvenient.